Multi-stage optical amplifier and broadband communication system

ABSTRACT

A multi-stage optical amplifier that has an optical fiber with a first length of amplifier fiber and a second length of amplifier fiber. The optical fiber is configured to be coupled to a signal source that produces at least a signal wavelength λ s  and a pump source that produces a pump wavelength λ p . Pump wavelength λ p  is less than signal wavelength λ s . Signal input, signal output and pump input ports are each coupled to the optical fiber. A first lossy member is coupled to the optical fiber and positioned between the first and second lengths of amplifier fiber. A pump shunt is coupled to the signal input port and the signal output port.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of ProvisionalApplication Ser. No. 60/089,426, filed Jun. 16, 1999 and acontinuation-in-part of application Ser. No.______ , identified asAttorney Docket No. 20434-701, both of which are fully incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to multi-stage opticalamplifiers, and more particularly to broadband communication systemsthat include one or more multi-stage optical amplifiers.

[0004] 2. Description of the Related Art

[0005] The demand for bandwidth continues to grow exponentially onfiber-optic superhighways due to applications such as datacommunications and the internet. Consequently, there is much effort atexploiting the bandwidth of optical fibers by using higher speeds perchannel. Examples include time-division multiplexed systems-andwavelength-division multiplexing (WDM).

[0006] Most fiber-optic networks currently deployed use standardsingle-mode fiber or dispersion-shifted fiber (DSF). Standard fiber hasa zero dispersion wavelength around 1310 nm, and the dispersion isprimarily resulting from the inherent glass dispersion. Currently, mostof the terrestrial network in the U.S. and the world is based onstandard fiber

[0007] With DSF, waveguide dispersion is used to shift the zerodispersion wavelength to longer wavelengths. A conventional DSF has azero dispersion wavelength at 1550 nm, coinciding with the minimum lossin a fused silica fiber. However, the zero dispersion wavelength can beshifted around by varying the amount of waveguide dispersion added. DSFis used exclusively in two countries, Japan and Italy, as well as in newlong-haul links.

[0008] The limiting factors for a fiber-optic transmission line includeloss, dispersion and gain equalization. Loss refers to the fact that thesignal attenuates as it travels in a fiber due to intrinsic scattering,absorption and other extrinsic effects such as defects. Opticalamplifiers can be used to compensate for the loss. Dispersion means thatdifferent frequencies of light travel at different speeds, and it comesfrom both the material properties and waveguiding effects. When usingmulti-wavelength systems and due the non-uniformity of the gain withfrequency, gain equalization is required to even out the gain over thedifferent wavelength channels.

[0009] The typical solution to overcoming these limitations is toperiodically place in a transmission system elements to compensate foreach of these problems. For example, a dispersion compensator can beused to cancel the dispersion, an optical amplifier used to balance theloss and a gain equalization element used to flatten the gain. Examplesof dispersion compensators include chirped fiber gratings and dispersioncompensating fiber (DCF). Examples of optical amplifiers includeerbium-doped fiber amplifiers (EDFAs), Raman amplifiers, and non-linearfiber amplifiers (NLFAs).

[0010] Another problem that arises in WDM systems is interaction orcross-talk between channels through non-linearities in the fiber. Inparticular, four-wave mixing (4WM) causes exchange of energy betweendifferent wavelength channels, but 4WM only phase matches near the zerodispersion wavelength. Consequently, if a fiber link is made fromconventional DSF, it is difficult to operate a WDM system from around1540-1560 nm. This turns out to be quite unfortunate because typicalEDFA's have gain from 1535-1565 nm, and the more uniform gain band isnear 1540-1560 nm. A second fiber nonlinearity that can be troublesomeis modulation instability (MI), which is 4WM where the fiber's nonlinearindex-of-refraction helps to phase match. However, MI only phase matcheswhen the dispersion is positive or in the so-called soliton regime.Therefore, MI can be avoided by operating at wavelengths shorter thanthe zero dispersion wavelength.

[0011] As the bandwidth utilization over individual fibers increases,the number of bands used for transmission increases. For WDM systemsusing a number of bands, additional complexities arise due tointeraction between and amplification in multi-band scenarios. Inparticular, particular system designs are needed for Raman amplificationin multi-band transmission systems. First, a new nonlinearity penaltyarises from the gain tilt from the Raman effect between channels. Thisarises because long wavelength channels tend to rob energy from theshort wavelength channels. Therefore, a means of minimizing the gaintilt on existing channels with the addition of new WDM channels isrequired.

[0012] To minimize both the effects of 4WM and Raman gain tilt, anothertechnical strategy is to use distributed Raman amplification. In a WDMsystem with multi-bands, a complexity arises from interaction betweenthe different pumps along the transmission line.

[0013] There is a need for greater bandwidth for broadband communicationsystems. A further need exists for broadband communication systems withreduced loss. Yet another need exists for broadband communicationsystems in the short wavelength region (S-band) covering the wavelengthrange of approximately 1430-1530 nm. Another need exists for broadbandcommunication systems with improved dispersion compensation.

SUMMARY OF THE INVENTION

[0014] Accordingly, it is an object of the present invention to provideimproved multi-stage optical amplifiers and broadband communicationsystems.

[0015] Another object of the present invention is to provide multi-stageoptical amplifiers and broadband communication systems with greaterbandwidth.

[0016] Yet another object of the present invention is to providemulti-stage optical amplifiers and broadband communication systems inthe S band.

[0017] A further object of the present invention is to providemulti-stage optical amplifiers and broadband communication systems thatuse standard fiber and DSF with different zero dispersion wavelengths.

[0018] Another object of the present invention is to provide amulti-stage optical amplifier and broadband communication system thatcombines the C and S bands.

[0019] Yet another object of the present invention is to providemulti-stage optical amplifiers and broadband communication systems thatcombine the C, S and L bands.

[0020] A further object of the present invention is to providemulti-stage optical amplifiers and broadband communication systems withgain tilt control

[0021] It is yet another object of the present invention to provide WDMsystems over DSF links by using the “violet” band in Raman amplifierswith dispersion compensating fiber to avoid nonlinearity limitationsfrom 4WM and MI.

[0022] These and other objects of the present invention are achieved ina multi-stage optical amplifier that has an optical fiber including afirst length of amplifier fiber and a second length of amplifier fiber.The optical fiber is configured to be coupled to a signal source thatproduces at least a signal wavelength λ_(s) and a pump source thatproduces a pump wavelength λ_(p). Pump wavelength λ_(p) is less thansignal wavelength λ_(s). Signal input, signal output and pump inputports are each coupled to the optical fiber. A first lossy member iscoupled to the optical fiber and positioned between the first and secondlengths of amplifier fiber. A pump shunt is coupled to the signal inputport and the signal output port.

[0023] In another embodiment, the present invention is a broadbandcommunication system with a transmitter and a receiver. An optical fiberis coupled to the transmitter and receiver. The optical fiber includesat least a first Raman amplifier fiber and a second Raman amplifierfiber. The optical fiber is configured to be coupled to at least onesignal source that produces at least a signal wavelength λ_(s) and atleast two pump sources that collectively produce a pump beam ofwavelength λ_(p). Pump wavelength λ_(p) is less than signal wavelengthλ_(s). Signal input, signal output and a first pump input port are eachcoupled to the optical fiber. The first Raman amplifier fiber ispositioned between the signal input port and the pump input port. Thesecond Raman amplifier fiber is positioned between the pump input portand signal output port. A second pump input port is coupled to theoptical fiber and positioned between the second Raman amplifier fiberand the signal output port. A first lossy member is positioned betweenthe pump input port and the signal output port. The lossy member islossy in at least one direction so that passage of the pump radiation ofwavelength λ_(p) from the second to the first length of amplifier fiberis substantially blocked.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic diagram of one embodiment of a multi-stageoptical amplifier of the present invention that includes a pump shunt.

[0025]FIG. 2 illustrates that the cutoff wavelength of the fiber usedwith the present invention should be shorter than the pump and signalwavelengths.

[0026]FIG. 3 is a schematic diagram illustrating the inclusion of adispersion compensating element, a gain equalization element and anadd/drop multiplexer to the multi-stage optical amplifier of the presentinvention.

[0027]FIG. 4 is a schematic diagram of another embodiment of amulti-stage optical amplifier of the present invention that includes twopump shunts.

[0028]FIG. 5 is a schematic diagram of another embodiment of amulti-stage optical amplifier of the present invention that includes apump shunt and four lengths of amplifier fiber.

[0029]FIG. 6 is a schematic diagram of one embodiment of a multi-stageoptical amplifier of the present invention that includes a pump shuntand two pump sources.

[0030]FIG. 7 is a schematic diagram of one embodiment of a multi-stageoptical amplifier of the present invention that includes a pump shuntand a circulator.

[0031]FIG. 8(a) is a schematic diagram of another embodiment of amulti-stage optical amplifier of the present invention that includes twolengths of Raman amplifier fiber and two pump sources. FIG. 8(b) is aschematic diagram of an embodiment of the present invention with adiscrete and a distributed amplifier; where distributed amplification isadded with only counter-propagating Raman pumps FIG. 8(c) is a schematicdiagram of an embodiment of the present invention similar to FIG. 8(b)in which mid-span access is not available but bi-directional pumping isallowed.

[0032]FIG. 9 is a schematic diagram of another embodiment of amulti-stage optical amplifier of the present invention that includesthree lengths of Raman amplifier fiber and three pump sources.

[0033]FIG. 10 is a schematic diagram illustrating four pump source whoseoutputs are combined using wavelength and polarization multiplexing.

[0034]FIG. 11 is a schematic diagram illustrating eight pump sourcewhose outputs are combined using wavelength and polarizationmultiplexing.

[0035]FIG. 12 is a schematic diagram illustrating that Brillouinthreshold for a laser diode pump source can be minimized with theinclusion of a spectrum broadening device.

[0036]FIG. 13 is a schematic diagram of a broadband booster amplifierembodiment of the present invention.

[0037]FIG. 14 is a schematic diagram of a broadband pre-amplifierembodiment of the present invention.

[0038]FIG. 15 is a schematic diagram of one embodiment of a broadbandcommunication system of the present invention.

[0039]FIG. 16 is a schematic diagram of another embodiment of abroadband communication system of the present invention.

[0040]FIG. 17 is a schematic diagram of another embodiment of abroadband communication system of the present invention.

[0041]FIG. 18 is a schematic diagram of another embodiment of abroadband communication system of the present invention.

[0042]FIG. 19 is a schematic diagram of another embodiment of abroadband communication system of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0043] One embodiment of the present invention, as illustrated in FIG.1, is a multi-stage optical amplifier 10 with an optical fiber 12including a first length of amplifier fiber 14 and a second length ofamplifier fiber 16. Optical fiber 12 is configured to be coupled to asignal source 18 that produces at least a signal wavelength λ_(s) and apump source 20 that produces a pump wavelength λ_(p). Pump wavelengthλ_(p) is less than signal wavelength λ_(s). Signal input port 22, signaloutput port 24 and pump input port 26 are each coupled to optical fiber12. A first lossy member 28 is coupled to optical fiber 12 andpositioned between the first and second lengths of amplifier fiber 14and 16 respectively. A pump shunt 30 is coupled to signal input port 22and signal output port 24. Optionally, a second lossy member 32 iscoupled to pump shunt 30. Pump shunt 30 can be an optical fiber that isintegral with optical fiber 12 or a separate optical fiber.

[0044] Pump beam λ_(p) propagates towards signal input port 22 fromfirst length of amplifier fiber 14 and away from signal input port 22 tosecond length of amplifier fiber 16.

[0045] First and second lengths of amplifier fiber 14 and 16 eachpreferably have a length greater than or equal to 200 m. Pump wavelengthλ_(p) is preferably in the range of 1300 nm to 1530 nm, and the signalwavelength can be in the range of 1430 to 1530 nm. Suitable pump sources20 include but are not limited to laser diodes (LD's), solid statelasers, fiber-based cascaded Raman wavelength shifters, cladding pumpedfiber lasers and the like.

[0046] First lossy member 28 can be an optical isolator, an add/dropmultiplexer. a gain equalization member, a dispersion compensationelement and the like. One or both of first and second lengths ofamplifier fiber 14 and 16 can be Raman amplifiers. Lossy elements 28 canalso be placed before and after first and second lengths of amplifierfiber 14 and 16 to prevent disturbance of amplifier performance fromspurious reflections from the transmission line. Additionally, a secondlossy element 32 can be inserted into pump shunt 30 to reduce themulti-path interference of the signal beam in amplifiers 12 and 14.

[0047] Additionally, one or both of first and second lengths ofamplifier fiber 14 and 16 can be implemented in dispersion compensatingfiber (DCF). A DCF is a fiber whose zero dispersion point is shifted towavelengths much longer than 1500 nm using the waveguide dispersionproperty. Consequently, DCF tend to have a small affective core area andsignificant germanium doping in the core, both of which lead to anenhancement of the Raman gain coefficient. DCF's are generally addedperiodically to a high-speed transmission link to compensate for thedispersion accumulated in the line.

[0048] In one embodiment, multi-stage optical amplifier 10 operates in aviolet band between 1430 and 1530 nm. Fiber 12 is a DSF with at leastone fiber non-linearity effect and a zero dispersion wavelength. In thisembodiment, multi-stage optical amplifier 10 provides gain in the violetband sufficiently far from the zero dispersion wavelength to avoidnon-linearity effects.

[0049] First length of amplifier fiber 14 preferably has lower noisethan second length of amplifier fiber 16. Second length of amplifierfiber 16 has a higher gain than first length of amplifier fiber 14. Inone embodiment, first length of amplifier fiber 14 has an optical noisefigure of less than 8 dB, and second length of amplifier fiber 16 has again level of at least 5 dB.

[0050] One or more WDM couplers 34 are used to couple a pump path fromthe signal input port 22 to the signal output port 24. WDM couplers 34are designed to pass (couple over) the signal band while coupling over(passing) the pump beams. Exemplary WDM couplers 34 includefused-tapered fiber couplers, Mach-Zehnder couplers, thin-filmdielectric filters, bulk diachronic elements and the like.

[0051] Signal input port 22 inputs signal λ_(s) which is amplifiedthrough Raman scattering when first and second lengths of amplifierfiber 14 and 16 are Raman amplifiers. The dispersion and length of thefirst and second lengths of amplifier fiber 14 and 16 can be selected tobe of the same magnitude of dispersion-length product as thetransmission link but of the opposite sign of dispersion. First andsecond lengths of amplifier fiber 14 and 16 are preferably made singlespatial mode for pump source 20 and signal wavelengths by making thecut-off wavelength of the gain fiber shorter than the pump wavelength.In particular, the cut-off wavelength is the wavelength below whichfirst and second lengths of amplifier fiber 14 and 16 support more thanone mode or becomes multi-mode. If the pump or signal falls into themulti-mode region, then additional noise arising from the beatingbetween different modes may arise.

[0052] As shown in FIG. 2 the fiber cut-off wavelength should be shorterthan the pump wavelength λ_(p). Pump wavelength λ_(p) is shorter thansignal wavelength λ_(s). Multi-stage optical amplifier 10 is pumped sothe net gain equals or exceeds the sum of losses in the transmissionlink and first and second lengths of amplifier fiber 14 and 16.

[0053]FIG. 3 illustrates that a dispersion compensating element 33, gainequalization element 29 or an add/drop multiplexer 31 can be includedand positioned between first and second lengths of amplifier fiber 14and 16.

[0054]FIG. 4 illustrates an embodiment of multi-stage optical amplifier10 with a third length of amplifier fiber 42. Second lossy member 32 ispositioned between second and third lengths of amplifier fiber 16 and42. A second pump shunt is coupled to second and third WDM couplers 46and 48. Additional lengths of amplifier fiber can also be included.

[0055] As illustrated in FIG. 5, multi-stage optical amplifier 10 caninclude a third and a fourth length of amplifier fiber 42 and 50,respectively. In this embodiment, third and fourth lengths of amplifierfiber 42 and 50 are coupled to pump shunt 30. Second lossy member 32 ispositioned between third and fourth lengths of amplifier fiber 42 and50.

[0056] In another embodiment of multi-stage optical amplifier 10,multiple pump sources are utilized. In FIG. 6, pump source 20 ispositioned between first length of amplifier fiber 14 and first lossymember 28. A second pump source 52 is positioned between second lengthof amplifier fiber 16 and signal output port 24 and is coupled to asecond pump input port 54. First pump source 20 produces a pump beam ofwavelength λ_(p1) and second pump source 52 produces 52 a pump beam ofwavelength λ_(p2). Wavelength λ_(p1) and wavelength λ_(p2) can be thesame or different. Pump sources 20 and 44 collectively produce a pumpbeam of wavelength λ_(p). Pump wavelength λ_(p) is less than a signalwavelength λ_(s).

[0057] In another embodiment, illustrated in FIG. 7, multi-stageamplifier 10 includes one or more circulators 56 to provide isolationbetween the first and second lengths of amplifier fiber 14 and 16.Circulator 56 also is useful as a means of dumping the remaining pumpwhich can be reused elsewhere for monitoring purposes.

[0058] As illustrated in FIG. 8(a), multi-stage optical amplifier 10 canhave an open loop configuration, In this embodiment, optical fiber 12 ispumped by a pump beam generated by pump sources 20 and 52 and first andsecond lengths of amplifier fiber 14 and 16 are each Raman amplifiers.Optical fiber 12 is preferably single spatial mode at both the signaland pump wavelengths. Again, wavelength λ_(p1) and wavelength λ_(p2) canbe the same or different. The pump beam has a wavelength shorter thanthe signal wavelengths. Pump sources 20 and 52 collectively produce apump beam of wavelength λ_(p). An amplified signal is then outputthrough signal output port 24. Pump sources 20 and 52 are coupled inthrough WDM couplers 34 and 58 which transmit signal wavelength λ_(s)over the pump wavelength λ_(p). First lossy member 28 is positionedbetween pump input port 26 and signal output port 24. In thisembodiment, the signal flows in a first direction and the pump beamflows in a reverse direction relative to the first direction. First andsecond lengths of amplifier fiber 14 and 16 are pumped in acounter-propagating manner. It may also be desirous to havebi-directional pumping in second length of amplifier fiber 16 toincrease the power amplifier gain without severely impacting the noisefigure of multi-stage optical amplifier 10.

[0059] Other elements, including but not limited dispersion compensatingelement 33, gain equalization element and add/drop multiplexer 31 may beincluded and positioned between first and second lengths of amplifierfiber 14 and 16.

[0060] In another embodiment, illustrated in FIGS. 8(b)-8(c), firstlength of amplifier fiber 14 is a distributed Raman amplifier fiber andsecond length of amplifier fiber 16 is a discrete Raman amplifier fiber.A distributed Raman amplifier fiber is an amplifier where at least somepart of the transmission link is pumped and involved in amplification.In this embodiment, first lossy member 28 is not positioned betweenfirst and second lengths of amplifier fiber 14 and 16. In FIG. 8(b)distributed amplification is added with only counter-propagating Ramanpumps. When access at a mid-point stage exists alternate band pumps areadded at different spatial points to minimize nonlinear interactionbetween pumps. In FIG. 8(c) mid-span access is not available butbi-directional pumping is allowed. The embodiment of FIG. 8(c) can beused where alternate band Raman pumps are launched in differentdirections in order to minimize interaction between pumps.

[0061] The open loop embodiment of multi-stage optical amplifier 10 canhave three or more lengths of amplifier fiber. Referring now to FIG. 9,an embodiment of multi-stage optical amplifier 10 is illustrated withthird length of amplifier fiber 42 coupled to a third pump source 60which is turn is coupled to a third pump input port 62. WDM coupler 64is coupled to third pump input port 62. Some or all of first, second andthird pump sources 20, 52 and 60 can be laser diode sources. Pump source60 produces a pump beam of wavelength λ_(p3). Wavelengths λ_(p1), λ_(p2)and λ_(p3) can be the same or different. Pump sources 20, 44 and 60collectively produce pump beam of wavelength λ_(p). An amplified signalis then output through signal output port 24.

[0062] As illustrated in FIGS. 10 and 11 each of pump source 20, 52 and60 can include multiple pump sources whose outputs can be combined usingwavelength and polarization multiplexing. Multiple combination ratings66 and PBS's 68 can be utilized. Additionally, some or all of themultiple pump sources which comprise pump sources 20, 52 and 60 can belaser diodes.

[0063] Referring now to FIG. 12, a spectrum broadening device 70 can becoupled to each pump source 20, 52 and 60. This is particularly usefulfor laser diode pump sources. Spectrum broadening device 70 broadens thespectrum while minimizing. Brillouin threshold. Suitable spectrumbroadening devices 70 include but are not limited to, (i) a grating thatis sufficiently broadband that can be chirped and cascade individualwavelengths, (ii) positioning a grating in a laser diode external cavityto cause appropriate line broadening and (iii) a dithering drive.Additionally pump pulsing can be used to broaden the spectrum.

[0064] The Brillouin threshold is reached when the following conditionis satisfied:

{tilde over (g_(B))}= P ₀ ^(LD) ·{fraction (L_(eff)/A_(eff))}≦18

[0065] where

[0066] P₀ ^(LD)=power of laser diode$L_{eff} = {{\frac{1}{\propto} \cdot \lbrack {1 - \exp^{- {\propto L}}} \rbrack}\quad {effective}\quad {pumping}\quad {length}}$

[0067] A_(eff) =effective area of fiber 12${\overset{\sim}{g}}_{B} = {\frac{\Delta \quad \gamma_{B}}{{\Delta \quad \gamma_{B}} + {\Delta \quad \gamma_{P}}} \cdot g_{B}}$${\overset{\sim}{g}}_{B} = {\frac{\Delta \quad \gamma_{B}}{{\Delta \quad \gamma_{B}} + {\Delta \quad \gamma_{P}}} \cdot g_{B}}$

[0068] Multi-stage optical amplifier 10 can be an in-line broadbandamplifier, a booster amplifier, a broadband pre-amplifier andincorporated in any variety of different broadband communicationsystems. In another embodiment, illustrated in FIG. 13, the presentinvention is a broadband booster amplifier 72 that includes amulti-stage optical amplifier 10 coupled to a transmitter 73.Transmitter 73 can include a WDM combiner 74 and a plurality oftransmitters 76. The plurality of transmitters 76 transmit a pluralityof wavelengths. The plurality of wavelengths may include at least afirst band of wavelengths and a second band of wavelengths. With thepresent invention, a variety of different transmitters 76 can beutilized including but not limited to laser diodes, tunable lasers, orbroadband sources such as continuum sources or light-emitting diodes.

[0069]FIG. 14 illustrates a broadband pre-amplifier embodiment of thepresent invention. Broadband pre-amplifier 78 includes multi-stageoptical amplifier 10 coupled to a receiver 80. Receiver 80 can include aWDM splitter 82 coupled to a plurality of receivers 84. Suitablereceivers 84 include but are not limited to germanium or InGaAs orInGaAsP detectors followed by electronics well known to those skilled inthe art.

[0070] In another embodiment, illustrated in FIG. 15, the presentinvention is a broadband communication system 86. In this embodiment,multi-stage optical amplifier 10 is an in-line broadband amplifier.Multi-stage optical amplifier 10 is coupled to one or more transmitters73 and one or more receivers 80.

[0071]FIG. 16 illustrates another embodiment of the present inventionwhich is a broadband communication system 88 that includes multi-stageoptical amplifier 10 coupled to a broadband pre-amplifier 90.Multi-stage optical amplifier 10 is coupled to one or more transmitters73 and broadband pre-amplifier 90 is coupled to one or more receivers80.

[0072]FIG. 17 illustrates yet another embodiment of a broadbandcommunication system 92 with a broadband booster amplifier 94 coupled tomulti-stage optical amplifier 10. One or more transmitters 73 is coupledto broadband booster amplifier 94. One or more receivers 80 is coupledto multi-stage optical amplifier 10.

[0073] Another embodiment of a broadband communication system 96 isillustrated in FIG. 18. In this embodiment, an in-line amplifier 98 iscoupled to receiver 80 and to a transmitter 100. Transmitter includesmulti-stage optical amplifier 10 coupled to transmitter 73.

[0074]FIG. 19 illustrates another broadband communication system 102 ofthe present invention. Broadband communication system 102 includesmulti-stage optical amplifier 10 coupled to broadband booster amplifier94 and broadband pre-amplifier 90. Broadband booster amplifier 94 iscoupled to one or more transmitters 73. Broadband pre-amplifier 90 iscoupled to one or more receivers 80.

[0075] While embodiments of the invention have been illustrated anddescribed, it is not intended that these embodiments illustrate anddescribe all possible forms of the invention. Rather, the words used inthe specification are words of description rather than limitation, andit is understood that various changes may be made without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A multi-stage optical amplifier, comprising: anoptical fiber including a first length of amplifier fiber and a secondlength of amplifier fiber, the optical fiber configured to be coupled toa signal source that produces at least a signal wavelength λ_(s) and apump source that produces a pump wavelength λ_(p), wherein pumpwavelength λ_(p) is less than signal wavelength λ_(s); a signal inputport coupled to the optical fiber; a signal output port coupled to theoptical fiber; a pump input port coupled to the optical fiber; a firstlossy member coupled to the optical fiber and positioned between thefirst and second lengths of amplifier fiber, the first lossy memberbeing lossy in at least one direction; and a pump shunt coupled to thesignal input port and the signal output port.
 2. The multi-stage opticalamplifier of claim 1, wherein the first and lengths of amplifier fibereach have a length greater than or equal to 200 m.
 3. The multi-stageoptical amplifier of claim 1, wherein pump radiation of wavelength λ_(p)is in the range of 1300 nm to 1530 nm
 4. The multi-stage opticalamplifier of claim 1, wherein signal radiation of wavelength λ_(s) is inthe range of 1430 to 1530 nm.
 5. The multi-stage optical amplifier ofclaim 1, wherein the first lossy member is an optical isolator.
 6. Themulti-stage optical amplifier of. claim 1, wherein the first lossymember is an add/drop multiplexer.
 7. The multi-stage optical amplifierof claim 1, wherein the first lossy member is a gain equalizationmember.
 8. The multi-stage optical amplifier of claim 1, wherein thefirst lossy member is a dispersion compensation element.
 9. Themulti-stage optical amplifier of claim 1, wherein the first length ofamplifier fiber is a Raman amplifier.
 10. The multi-stage opticalamplifier of claim 1, wherein the second length of amplifier fiber is aRaman amplifier.
 11. The multi-stage optical amplifier of claim 1,wherein at least one of the first and second Raman fiber amplifiers is adispersion compensating fiber.
 12. The multi-stage optical amplifier ofclaim 11, wherein the first and Raman fiber amplifiers are eachdispersion compensating fibers.
 13. The multi-stage optical amplifier ofclaim 1, wherein the first length of amplifier fiber has lower noisethan the second length of amplifier fiber.
 14. The multi-stage opticalamplifier of claim 1, wherein the second length of amplifier fiber has ahigher gain than the first length of amplifier fiber.
 15. Themulti-stage optical amplifier of claim 1, further comprising: at leastone WDM coupler to couple a pump path from the signal input port to thesignal output port.
 16. The multi-stage optical amplifier of claim 1,wherein the first length of amplifier fiber has an optical noise figureof less than 8 dB.
 17. The multi-stage optical amplifier of claim 1,wherein the second length of amplifier fiber has a gain level of atleast 5 dB.
 18. The multi-stage optical amplifier of claim 1, furthercomprising: a pump source coupled to the pump input port.
 19. Themulti-stage optical amplifier of claim 1, further comprising: at leastone laser diode pump source coupled to the pump input port.
 20. Themulti-stage optical amplifier of claim 1, further comprising: a secondlossy member coupled to the pump shunt.
 21. The multi-stage opticalamplifier of claim 1, wherein the pump shunt includes an optical fiber.22. A broadband booster amplifier, comprising: a plurality oftransmitters transmitting a plurality of wavelengths; a combiner coupledto the plurality of transmitters; an optical fiber coupled to thecombiner, the optical fiber including a first length of amplifier fiberand a second length of amplifier fiber, the optical fiber configured tobe coupled to a signal source and a pump source; a signal input portcoupled to the optical fiber; a signal output port coupled to theoptical fiber; a pump input port coupled to the optical fiber; a firstlossy member coupled to the optical fiber and positioned between thefirst and second lengths of amplifier fiber, the first lossy memberbeing lossy in at least one direction; and a pump shunt coupled to thesignal input port and the signal output port.
 23. A broadbandpre-amplifier, comprising: an optical fiber including a first length ofamplifier fiber and a second length of amplifier fiber, the opticalfiber configured to be coupled to a signal source and a pump source; asignal input port coupled to the optical fiber; a signal output portcoupled to the optical fiber; a pump input port coupled to the opticalfiber; a first lossy member coupled to the optical fiber and positionedbetween the first and second lengths of amplifier fiber, the first lossymember being lossy in at least one direction; a pump shunt coupled tothe signal input port and the signal output port; a splitter coupled tothe signal output port; and a plurality of receivers coupled to thesplitter.
 24. A broadband communication system, comprising: atransmitter; an optical fiber including a first length of amplifierfiber and a second length of amplifier fiber, the optical fiberconfigured to be coupled to a signal source and a pump source; a signalinput port coupled to the optical fiber; a signal output port coupled tothe optical fiber; a pump input port coupled to the optical fiber; afirst lossy member coupled to the optical fiber and positioned betweenthe first and second lengths of amplifier fiber, the first lossy memberbeing lossy in at least one direction; a pump shunt coupled to thesignal input port and the signal output port; and a receiver coupled tothe optical fiber.
 25. The system of claim 24, wherein the first andlengths of amplifier fiber each have a length greater than or equal to200 m.
 26. The system of claim 24, further comprising: a pump sourceproducing radiation of wavelength λ_(p) in the range of 1300 nm to 1530nm
 27. The system of claim 24, further comprising: a signal sourceproducing radiation of wavelength λ_(s) in the range of 1430 to 1530 nm.28. The system of claim 24, wherein the first lossy member is an opticalisolator.
 29. The system of claim 24, wherein the first lossy member isan add/drop multiplexer.
 30. The system of claim 24, wherein the firstlossy member is a gain equalization member.
 31. The system of claim 24,wherein the first lossy member is a dispersion compensation element. 32.The system of claim 24, wherein the first length of amplifier fiber is aRaman amplifier.
 33. The system of claim 24, wherein the second lengthof amplifier fiber is a Raman amplifier.
 34. The system of claim 24,wherein at least one of the first and second Raman fiber amplifiers is adispersion compensating fiber.
 35. The system of claim 34, wherein thefirst and Raman fiber amplifiers are each dispersion compensatingfibers.
 36. The system of claim 24, wherein the first length ofamplifier fiber has lower noise than the second length of amplifierfiber.
 37. The system of claim 24, wherein the second length ofamplifier fiber has a higher gain than the first length of amplifierfiber.
 38. The system of claim 24, further comprising: at least one WDMcoupler to couple a pump path from the signal input port to the signaloutput port.
 39. The system of claim 24, wherein the first length ofamplifier fiber has an optical noise FIG. of less than 8 dB.
 40. Thesystem of claim 24, wherein the second length of amplifier fiber has again level of at least 5 dB.
 41. The system of claim 24, furthercomprising: a laser diode pump source coupled to the pump input port.42. The system of claim 24, further comprising: a second lossy membercoupled to the pump shunt.
 43. The system of claim 24, wherein the pumpshunt includes an optical fiber.
 44. A broadband communication system,comprising: a transmitter; an optical fiber coupled to the transmitter,the optical fiber including a first length of amplifier fiber and asecond length of amplifier fiber, the optical fiber configured to becoupled to a signal source and a pump source; a signal input portcoupled to the optical fiber; a signal output port coupled to theoptical fiber; a pump input port coupled to the optical fiber; a firstlossy member coupled to the optical fiber and positioned between thefirst and second lengths of amplifier fiber, the first lossy memberbeing lossy in at least one direction; a pump shunt coupled to thesignal input port and the signal output port; at least one in-linebroadband amplifier coupled to the optical fiber; and a receiver coupledto the in-line broadband amplifier.
 45. The system of claim 44, whereinthe in-line broadband amplifier comprises: an optical fiber including afirst length of amplifier fiber and a second length of amplifier fiber,the optical fiber configured to be coupled to a signal source and a pumpsource; a signal input port coupled to the optical fiber; a signaloutput port coupled to the optical fiber; a pump input port coupled tothe optical fiber; a first lossy member coupled to the optical fiber andpositioned between the first and second lengths of amplifier fiber, thefirst lossy member being lossy in at least one direction; and a pumpshunt coupled to the signal input port and the signal output port.
 46. Abroadband communication system, comprising: a transmitter; a broadbandbooster amplifier; an optical fiber coupled to the broadband boosteramplifier, the optical fiber including a first length of amplifier fiberand a second length of amplifier fiber, the optical fiber configured tobe coupled to a signal source and a pump source; a signal input portcoupled to the optical fiber; a signal output port coupled to theoptical fiber; a pump input port coupled to the optical fiber; a firstlossy member coupled to the optical fiber and positioned between thefirst and second lengths of amplifier fiber, the first lossy memberbeing lossy in at least one direction; a pump shunt coupled to thesignal input port and the signal output port; and a receiver coupled tothe optical fiber.
 47. The system of claim 46, wherein the broadbandbooster amplifier comprises: a plurality of transmitters transmitting aplurality of wavelengths; a combiner coupled to the plurality oftransmitters; an optical fiber coupled to the combiner, the opticalfiber including a first length of amplifier fiber and a second length ofamplifier fiber, the optical fiber configured to be coupled to a signalsource and a pump source; a signal input port coupled to the opticalfiber; a signal output port coupled to the optical fiber; a pump inputport coupled to the optical fiber; a first lossy member coupled to theoptical fiber and positioned between the first and second lengths ofamplifier fiber, the first lossy member being lossy in at least onedirection; and a pump shunt coupled to the signal input port and thesignal output port.
 48. A broadband communication system, comprising: atransmitter; an optical fiber coupled to the transmitter, the opticalfiber including a first length of amplifier fiber and a second length ofamplifier fiber, the optical fiber configured to be coupled to a signalsource and a pump source; a signal input port coupled to the opticalfiber; a signal output port coupled to the optical fiber; a pump inputport coupled to the optical fiber; a first lossy member coupled to theoptical fiber and positioned between the first and second lengths ofamplifier fiber, the first lossy member being lossy in at least onedirection; a pump shunt coupled to the signal input port and the signaloutput port; a broadband pre-amplifier coupled to the optical fiber; anda receiver coupled to the broadband pre-amplifier.
 49. The system ofclaim 48, wherein the broadband pre-amplifier comprises: an opticalfiber including a first length of amplifier fiber and a second length ofamplifier fiber, the optical fiber configured to be coupled to a signalsource and a pump source; a signal input port coupled to the opticalfiber; a signal output port coupled to the optical fiber; a pump inputport coupled to the optical fiber; a first lossy member coupled to theoptical fiber and positioned between the first and second lengths ofamplifier fiber, the first lossy member being lossy in at least onedirection; a pump shunt coupled to the signal input port and the signaloutput port; a splitter coupled to the signal output port; and aplurality of receivers coupled to the splitter.
 50. A broadbandcommunication system, comprising: a transmitter; a broadband boosteramplifier coupled to the transmitter; an optical fiber coupled to thebooster broadband amplifier, the optical fiber including a first lengthof amplifier fiber and a second length of amplifier fiber, the opticalfiber configured to be coupled to a signal source and a pump source; asignal input port coupled to the optical fiber; a signal output portcoupled to the optical fiber; a pump input port coupled to the opticalfiber; a first lossy member coupled to the optical fiber and positionedbetween the first and second lengths of amplifier fiber, the first lossymember being lossy in at least one direction; a pump shunt coupled tothe signal input port and the signal output port; a broadbandpre-amplifier coupled to the optical fiber; and a receiver coupled tothe broadband pre-amplifier.
 51. The system of claim 50, wherein thebroadband booster amplifier comprises: a plurality of transmitterstransmitting a plurality of wavelengths; a combiner coupled to theplurality of transmitters; an optical fiber coupled to the combiner, theoptical fiber including a first length of amplifier fiber and a secondlength of amplifier fiber, the optical fiber configured to be coupled toa signal source and a pump source; a signal input port coupled to theoptical fiber; a signal output port coupled to the optical fiber; a pumpinput port coupled to the optical fiber; a first lossy member coupled tothe optical fiber and positioned between the first and second lengths ofamplifier fiber, the first lossy member being lossy in at least onedirection; and a pump shunt coupled to the signal input port and thesignal output port.
 52. The system of claim 51, wherein the broadbandpre-amplifier comprises: an optical fiber including a first length ofamplifier fiber and a second length of amplifier fiber, the opticalfiber configured to be coupled to a signal source and a pump source; asignal input port coupled to the optical fiber; a signal output portcoupled to the optical fiber; a pump input port coupled to the opticalfiber; a first lossy member coupled to the optical fiber and positionedbetween the first and second lengths of amplifier fiber, the first lossymember being lossy in at least one direction; a pump shunt coupled tothe signal input port and the signal output port; a splitter coupled tothe signal output port; and a plurality of receivers coupled to thesplitter.